1 Process Description and Control Chapter 3

2 Process A program in execution An instance of a program running on a computer The entity that can be assigned to and executed on a processor A unit of activity characterized by the execution of a sequence of instructions, a current state, and an associated set of system instructions

3 Requirements of an Operating System Interleave the execution of multiple processes to maximize processor utilization while providing reasonable response time Allocate resources to processes Support inter-process communication and user creation of processes

4 First Thought of the Day Would we want applications to be written directly for a given hardware platform? –Compiler -> Hardware specific –For a general application we don’t want it hardware specific because there’s lots of different hardware out there. –A hardware driver -> For a specific hardware. –For consoles we don’t have to since hardware constant –Efficiency vs. Generality

5 Concepts Computer platform consists of a collection of hardware resources Computer applications are developed to perform some task Operating system provides a convenient to use, feature rich, secure, and consistent interface for applications to use OS provides a uniform, abstract representation of resources that can be requested and accessed by application

6 Second Thought for the Day How might a process get created? –The user starting the application –The system itself starts a process –A process can start another process

7 Process Creation

8 Third Thought of the Day What would cause a process to be terminated? –An uncaught or handled error within the program. –It finished what ever it was doing. –The user decides to end it. –The operating system.

9 Process Termination

10 Process Termination

11 Example Execution

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13 Fourth Thought of the Day What things to keep track of for a process? –Keep track of what ever was in the registers –How long the process has been running –Where its stored in memory so no one else over writes it. –How much its using the CPU –Priority –Identifier

14 Process Control Block

15 Process Control Block Contains the process elements Created and manage by the operating system – Specifically the Dispatcher Allows support for multiple processes

16 Two-State Process Model Process may be in one of two states –Running –Not-running

17 Not-Running Process in a Queue What is the problem with this simple process scheduling model? No priority No difference between those ready and those waiting for I/O.

18 Five-State Process Model

19 Using Two Queues Is one blocked queue enough and why (Hint: Do I/O devices all take the same amount of time)?

20 Multiple Blocked Queues

21 One Suspend State

22 Suspended Processes Processor is faster than I/O so all processes could be waiting for I/O Swap these processes to disk to free up more memory Blocked state becomes suspend state when swapped to disk

23 Reasons for Process Suspension

24 Two Suspend States

25 Process Control Block a Detailed Look

26 Process Control Block Process identification –Identifiers Numeric identifiers that may be stored with the process control block include –Identifier of this process –Identifier of the process that created this process (parent process) –User identifier

27 Process Control Block Processor State Information –User-Visible Registers A user-visible register is one that may be referenced by means of the machine language that the processor executes while in user mode. Typically, there are from 8 to 32 of these registers, although some RISC implementations have over 100.

28 Process Control Block Processor State Information –Control and Status Registers These are a variety of processor registers that are employed to control the operation of the processor. These include Program counter: Contains the address of the next instruction to be fetched Condition codes: Result of the most recent arithmetic or logical operation (e.g., sign, zero, carry, equal, overflow) Status information: Includes interrupt enabled/disabled flags, execution mode

29 Pentium II EFLAGS Register

30 Process Control Block Processor State Information –Stack Pointers Each process has one or more last-in-first-out (LIFO) system stacks associated with it. A stack is used to store parameters and calling addresses for procedure and system calls. The stack pointer points to the top of the stack.

31 Process Control Block Process Control Information –Scheduling and State Information This is information that is needed by the operating system to perform its scheduling function. Typical items of information: Process state: defines the readiness of the process to be scheduled for execution (e.g., running, ready, waiting, halted). Priority: One or more fields may be used to describe the scheduling priority of the process. In some systems, several values are required (e.g., default, current, highest-allowable)

32 Process Control Block Process Control Information –Scheduling and State Information Scheduling-related information: This will depend on the scheduling algorithm used. Examples are the amount of time that the process has been waiting and the amount of time that the process executed the last time it was running. Event: Identity of event the process is awaiting before it can be resumed

33 Process Control Block Process Control Information –Data Structuring How the processes may be organized (e.g. a queue, ring, or some other structure). –For example, all processes in a waiting state for a particular priority level may be linked in a queue. A process may exhibit a parent-child (creator- created) relationship with another process.

34 Process Control Block Process Control Information –Inter-process Communication Various flags, signals, and messages may be associated with communication between two independent processes. Some or all of this information may be maintained in the process control block. –Process Privileges Processes are granted privileges in terms of the memory that may be accessed and the types of instructions that may be executed. In addition, privileges may apply to the use of system utilities and services.

35 Process Control Block Process Control Information –Memory Management This section may include pointers to segment and/or page tables that describe the virtual memory assigned to this process. –Resource Ownership and Utilization Resources controlled by the process may be indicated, such as opened files. A history of utilization of the processor or other resources may also be included; this information may be needed by the scheduler.

36 Example: UNIX

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39 UNIX SVR4 Process Management Most of the operating system executes within the environment of a user process

40 Two Special Processes Process 0 : sched – Creates the init process. Process 1 : init – Is the parent to all other processes.

41 Steps to Create a Process An existing process makes a fork call to create the new process. ID created, Memory setup, and Process moved to ready to run state. Parent informed of child’s ID. Before the child process starts both the parent and the child are at the same line of code.

42 Chapter Summary

43 Process Creation Assign a unique process identifier Allocate space for the process Initialize process control block Set up appropriate linkages –Ex: add new process to linked list used for scheduling queue Create of expand other data structures –Ex: maintain an accounting file

44 When to Switch a Process Clock interrupt –process has executed for the maximum allowable time slice I/O interrupt Memory fault –memory address is in virtual memory so it must be brought into main memory

45 When to Switch a Process Trap –error or exception occurred –may cause process to be moved to Exit state Supervisor call –such as file open

46 Change of Process State Save context of processor including program counter and other registers Update the process control block of the process that is currently in the Running state Move process control block to appropriate queue – ready; blocked; ready/suspend Select another process for execution

47 Change of Process State Update the process control block of the process selected Update memory-management data structures Restore context of the selected process

48 End

49 Execution of the Operating System Non-process Kernel –Execute kernel outside of any process –Operating system code is executed as a separate entity that operates in privileged mode Execution Within User Processes –Operating system software within context of a user process –Process executes in privileged mode when executing operating system code

50 Execution of the Operating System Process-Based Operating System –Implement operating system as a collection of system processes –Useful in multi-processor or multi- computer environment

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52 UNIX Process States

53 UNIX Process Image